PMR is capable of delivering increased storage density as compared to longitudinal magnetic recording (LMR). Today there is interest in using PMR for hard disk drives (HDD). Current HDDs with LMR technology have an estimated limit of 100 to 200 gigabit per square inch due to the superparamagnetic effect. PMR technology is predicted to allow information densities of up to around 1 terabit per square inch (1000 gigabit per square inch).
One major challenge in designing magnetic information storage media is retaining the magnetization of the medium despite thermal fluctuations caused by the superparamagnetic limit. If the thermal energy is too high, there may be enough energy to reverse the magnetization in a region of the medium, destroying the data stored there. The energy required to reverse the magnetization of a magnetic region is proportional to the size of the magnetic region (where a larger magnetic region is more stable), as well as to the magnetic coercivity of the material. There is a minimum size for a magnetic region at a given temperature and coercivity. If it is any smaller it is likely to be randomly de-magnetized.
PMR uses higher coercivity material. This is possible because the head's write field penetrates the medium more efficiently in the perpendicular geometry. Hence, one advantage of PMR over LMR is that it achieves higher storage densities by aligning the poles of the magnetic elements, (which represent bits), perpendicularly to the surface of the disk platter.
FIGS. 1 and 2 are block diagrams showing PMR and LMR technologies for comparison. FIGS. 1 and 2 illustrate one advantage of PMR over LMR. FIG. 1 includes an LMR system 102 and FIG. 2 includes a PMR system 104. The LMR system 102 aligns bits horizontally on a storage layer 108. A writing element 103 (or write head) is responsible for determining the direction of each bit in the storage layer (either right or left), so that it can be used to represent a “0” or a “1” in binary notation.
The PMR system 104 aligns the bits perpendicularly on the storage layer 106. In the PMR system 104, the bits can be placed closer together on the storage layer 106, thus increasing the number of magnetic elements that can be stored in a given area.
The actual advantage of PMR is more complex, having to do with the use of a magnetically “stronger” (higher coercivity) material as the storage layer 106. This is possible due to the fact that in a perpendicular arrangement the magnetic flux 150 is guided through a magnetically soft (and relatively thick) underlayer 110 (or keeper layer) considerably thickening the total disk structure. This magnetically soft underlayer 110 can be effectively considered a part of the write head 112, making the write head 112 more efficient. Thus, the PMR system 104 makes it possible to produce a stronger write field gradient with essentially the same head materials as in the LMR system 102. Therefore, the PMR system 104 allows for the use of the higher coercivity storage layer 106.
A higher coercivity storage layer 106 is inherently thermally more stable, as stability is proportional to the product of bit (or magnetic grain) volume times the uniaxial anisotropy constant Ku, which in turn is higher for a material with a higher magnetic coercivity.
There are two essential spacing parameters in PMR, clearance or head media spacing (HMS) and head keeper spacing (HKS) although only HMS is defined in LMR. HMS measures the amount of space between the write head 112 and the storage layer 106. HKS measures the amount of space between the write head 112 and the soft magnetic underlayer 110. Typically, there is a non-magnetic interlayer 117 between the storage layer 106 and the underlayer 110 and its thickness comprises the HKS measurement. HKS is measured by varying HMS, for example, using a HMS varying module 152 as will be further defined subsequently.
There are known spacing loss equations that are used to measure HMS in an LMR technology. Current schemes in PMR seek to use the same spacing loss equations as approximations to measure HMS in PMR technologies as well. One basic difficultly, however, in using LMR based spacing loss equations as approximations to measure HMS in PMR is the limited range of wavelength where the approximation works well.
Traditional LMR technologies employ a measurement to be obtained by writing a test pattern to the storage layer, reading back the pattern, and analyzing the harmonic components of the pattern. The analysis of the harmonic components of the pattern comprises taking the amplitude ratio between any harmonic components for the sake of cancelling out the fluctuations caused by variables other than the spacing, such as media magnetic variation, off-track during the measurement, pre-amplification gain, or head sensitivity change over temperature, for example.
Thus, it has been common to write a rather long wavelength single tone and to take the ratio between first to third harmonic components in LMR. For example, a single tone may be written with a frequency of 6T (e.g., the tone has a frequency equivalent to six clock cycles) and taking the amplitude ratio between 6T and 2T.
The contribution of the additional media parameters of thickness of the storage layer 106 (d) and of the thickness of the interlayer (t) to the spacing loss equations in PMR becomes less significant towards a shorter wavelength. Conventional methods to measure spacing in PMR then take the ratio of harmonics within the short wavelength range.
One known solution was to take the ratio between the higher harmonics of a long wavelength single tone. This method works fairly well to estimate HMS by taking the ratio of amplitude difference between the third and fifth harmonics, such as taking a ratio between (10/3)T and 2T of a 10T single tone. This method, however, suffers from sensitivity to noise in the measurement system because the amplitude of the fifth harmonic is small. Only expensive equipment such as the spectrum analyzer can overcome the difficulty in analyzing the signal in this manner.
Another solution was not to use the single tone but to write the pattern “111100” in non-return-to-zero (NRZ) notation to the magnetic storage layer, to read back the pattern, and to analyze the 3T and 1.5T harmonics in the read back pattern. Noise is a problem with this method also, however. The 1.5T amplitude harmonic is usually lower than the 3T amplitude harmonic by more than 12 decibels and tends to be sensitive to the noise. This problem can only be alleviated as before by expensive equipment (such as a spectrum analyzer) for the analysis required to perform the measurement. Neither of the two methods above mentioned work for HKS measurements due to the absence of a lower harmonic component that contains the most amount of contribution of the soft underlayer (keeper) during the write process.
One solution in the above methods to obtain the HKS is to write an additional pattern to a separate portion of the magnetic storage layer or to re-write a separate pattern to the same portion of the magnetic layer, and analyze it separately. This solution is inadequate, however, because the magnetic storage layer is never perfectly uniform and never magnetized perfectly. Therefore, local variations in the magnetic storage layer will make the HMS and HKS measurements less accurate when measured using separate patterns on separate portions of the magnetic storage layer. Therefore, what is needed is a system and method that reduces or overcomes these significant problems found in the conventional solutions as described above.